As to a roller bearing for use in a clean oil or an oil contaminated with foreign substances (extraneous matter), or in grease lubrication, it is a common practice to subject a raceway member (race) thereof to a surface heat-treatment/hardening process, such as carburizing, nitriding or induction hardening, in order to extend the service life of a race thereof (see, for example, Japanese Unexamined Patent Publication No. 5-195070). For the purpose of increasing the longevity of the bearing used in the clean oil, in particular, various attempts have been made to increase the strength of the race by decreasing a content of a non-metal inclusion in the material, adding an alloy component or the like (see, for example, Japanese Unexamined Patent Publication No. 2002-220638). However, all of the aforementioned measures have a drawback of increasing the manufacture costs of the roller bearing.
Steels for bearing manufacture, such as bearing steels and carburized steels, have been used as a common material for the raceway member of the roller bearing. However, the steel for bearing manufacture is so expensive that the manufacture cost of the roller bearing is high. On this account, some bearing manufacturers adopt a method wherein the raceway member is formed from a relatively less expensive carbon steel for machine structural use, such as JIS-S45C and JIS-S55C. In this case, however, the race is inferior in the quenched hardness as compared with that of the steel for bearing manufacture, failing to achieve a sufficient fatigue strength. Hence, the raceway member suffers a short service life.
In some medium- or large-sized roller bearings having an outside diameter of 240 mm or more, on the other hand, a blank for raceway member is heat-treated and then, the race is finished to predetermined precisions by turning rather than by grinding. In this case, however, the resultant race has such a great surface roughness (e.g., a center-line average roughness Ra of 0.35 μm or more) that the race inevitably encounters boundary lubrication at its contact portion with a rolling element. Furthermore, the race is subjected to the heavy cutting process, which involves a potential risk of producing tensile residual stress at the race. Therefore, if the race is used in contact with the rolling element at high surface contact pressure, the race is more susceptible to premature flaking. In an attempt at further improving the finished surface (e.g., Ra≧0.2 μm) of the race formed by the aforesaid cutting process, the cutting conditions may be loosened (the race cut relatively lightly) or otherwise, the finished surface may be subjected to rough grinding. Such approaches also encounter the aforementioned problems.
In view of the foregoing, the invention has been accomplished and has an object to provide a method for manufacturing a bearing raceway member, which can extend the service life of the bearing raceway member at low costs.